This application is related to Japanese application No. 2001-140142 filed on May 10, 2001, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a magneto-optical recording medium (hereinafter may be referred to as xe2x80x9ca mediumxe2x80x9d). More specifically, it relates to a magneto-optical recording medium including a magneto-optical disc, tape, card and the like applied to a magneto-optical recording/reproducing device.
2. Description of Related Art
As an external recording device for computers, attention has recently been focused on a magneto-optical recording medium such as a magneto-optical disc. On the magneto-optical recording medium, recording bits in a submicron order are formed by applying an external magnetic field and irradiating a laser beam. Therefore, a recording capacitance of the magneto-optical recording medium can be increased as compared with conventional external recording media such as a floppy disc and a hard disc.
For example, a currently utilized 3.5 inch magneto-optical disc is provided with tracks at 0.9 xcexcm pitch within a radius of about 24-40 mm and recording marks of 0.38 xcexcm at the minimum are formed along a circumference direction, so that a recording capacitance of about 1.3 GB is obtained per one side of the disc. This medium reproduces data with a magnetically induced super-resolution technology, which is capable of reproducing recording marks smaller than beam spots.
More specifically, there has been known a technique for reading recording marks not exceeding a diffraction limit by reproducing only in a moderate temperature region while using a low temperature region and a high temperature region in a beam spot as masks. The technique is called as double mask RAD system (Japanese Unexamined Patent Publication No. Hei 7(1995)-244877).
However, in view of future multimedia generation, it is necessary to further increase the recording density to record enormous data and animation. In order to raise the recording density, more recording marks must be formed on the medium and therefore it is required to shorten the mark length and the distance between the marks.
To achieve the high density recording in such a manner, it may be possible to reduce a wavelength of a laser beam to be applied smaller than a currently used wavelength of 780 nm or 680 nm. However, in view of practical application, it is more effective to shorten the mark length rather than the laser wavelength.
For reading the short recording marks with a laser beam of unchanged wavelength, there is a method of raising NA (number of aperture) of an objective lens. With the raise of the NA, resolution capability increases so that much smaller marks can be read out. For example, an objective lens for the above-mentioned magneto-optical disc having the recording capacitance of about 1.3 GB has the NA of 0.55. By raising the NA to 0.8, the recording capacitance can be increased.
However, the more the NA increases, the more a thickness of a substrate needs to be reduced because a focus distance becomes small in accordance with the increase of the NA. Accordingly, where an objective lens having the NA greater than a conventional value is used, high capacitance can effectively be realized with a medium suitable for recording/reproducing through light irradiation on a magnetic film side of the medium, rather than with a medium suitable for recording/reproducing through light irradiation on a substrate side of the medium. In the former medium, films comprising the medium are formed in an order completely opposite to an order of forming films in the latter conventional medium.
In the above-described medium having a film structure opposite from that of the conventional medium, a thickness and a material of a layer below the magnetic film are different as compared with the conventional medium. Therefore, magnetic characteristics of the magnetic film are utterly different from those of the conventional medium, which deteriorates recording/reproducing characteristics.
An object of the present invention is to provide a magneto-optical recording medium which exhibits high recording/reproducing characteristics, is capable of improving the recording density, and particularly advantageous to the use of an objective lens of high NA even if its layered structure is in the opposite order from that of the conventional medium.
According to the present invention, provided is a magneto-optical recording medium for recording and reproducing data through light irradiation on a magnetic film side of the medium, the medium comprising at least one magnetic film which is formed on a substrate and has an easy axis of magnetization in a perpendicular direction and an in-plane magnetic film which is formed between the substrate and the magnetic film and has an easy axis of magnetization in an in-plane direction.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.